Hydrogen peroxide is useful as a basic chemical indispensable to the food, medicine, pulp, textile and semiconductor industries. Hitherto, hydrogen peroxide has been industrially produced by the anthraquinone process, which is a chemical process. However, there is a growing desire for an on-site hydrogen peroxide production apparatus. This is because hydrogen peroxide is an unstable substance difficult to store for long periods of time and from the standpoints of safety in transportation and pollution abatement.
In power plants and factories where seawater is utilized as cooling water, a technique for preventing organisms from attaching to the inside of a condenser has been employed which comprises directly electrolyzing seawater to generate hypochlorous acid, and utilizing its action on organisms to inhibit their attachment. However, restrictions are being placed on use of this technique from the standpoint of environmental conservation. This is because hypochlorous acid may react with marine organisms and organic substances present in seawater to form chlorinated organic substances in the seawater, and these reaction products in turn may cause secondary pollution.
On the other hand, it has been reported that addition of a minute amount of hydrogen peroxide to cooling water is sufficiently effective in preventing the attachment of organisms. It has further been reported that the addition of hydrogen peroxide is also effective in maintaining water in fish breeding farms. However, there are still problems concerning safety in transportation and pollution abatement as discussed above.
Investigative reports have hitherto been made on hydrogen peroxide synthesis techniques based on the reduction reaction of oxygen gas. U.S. Pat. No. 3,693,749 proposes several electrolysis apparatuses, while U.S. Pat. No. 4,384,931 discloses an electrolytic process for producing an alkaline hydrogen peroxide solution with an ion-exchange membrane. U.S. Pat. No. 3,969,201 discloses a hydrogen peroxide production apparatus including a carbon cathode having a three-dimensional structure and an ion-exchange membrane. However, the hydrogen peroxide solution thus obtained has limited use because the alkali concentration is too high for the concentration of hydrogen peroxide.
On the other hand, U.S. Pat. Nos. 4,406,758, 4,891,107 and 4,457,953 disclose methods in which a porous diaphragm material and a hydrophobic carbon cathode are used. In these methods, however, the operation is troublesome because the amount of electrolyte solution moving from the anode chamber to the cathode chamber, or the rate of movement, is difficult to control.
In the Journal of the Electrochemical Society, Vol.130, pp.1117--(1983), a method for stably obtaining an acidic hydrogen peroxide solution is proposed using a cation--and anion-exchange membrane while sulfuric acid is supplied to an intermediate chamber.
It has further been reported in Denki Kagaku, Vol.57, p.1073 (1989) that performance is improved by using united membrane electrodes as an anode. However, this technique is disadvantageous in cost because the electric power consumption is too high, and a fully satisfactory electrolysis apparatus based thereon has not yet been obtained.
These methods for hydrogen peroxide generation each is effective when the target compound is produced in an environment of an aqueous alkali solution. It is therefore necessary to supply an alkali ingredient as a raw material, and this also poses a transportation problem.
On the other hand, in view of the aforementioned problem associated with direct seawater electrolysis, it is certain that the use of hydrogen peroxide for seawater treatment is desirable from the standpoint of cost, and various investigations are being made thereon.
Among such techniques, the use of commercial hydrogen peroxide solutions may pose a problem with respect to adding to seawater chemicals which have not been derived from seawater itself, besides the problems described above. Namely, the addition of a synthetic chemical which has not been derived from the seawater itself may give rise to an environmental problem of contaminating the seawater itself. As a matter of course, if an alkali is externally supplied for alkali electrolysis for generating hydrogen peroxide, this may pose the same problem.
In order to avoid these problems, the present inventors previously proposed a method comprising subjecting seawater to salt separation to obtain an alkali, subsequently obtaining an aqueous hydrogen peroxide solution, and neutralizing the alkali with an acid separated from the seawater to thereby enable treatment with hydrogen peroxide. This method is nearly ideal in that environmental problems are minimal because there is absolutely no need to added an external chemical, and in that the amount of electric power required is exceedingly small. However, from the standpoint of actually conducting continuous electrolysis, there has been a need to completely remove calcium and magnesium from the seawater to obtain increased efficiency. There has also been a need for an electrolysis apparatus which has a simpler structure and which can be handled more easily.